The invention relates to a process for forming protecting coatings on zinc surfaces. It also relates to compositions used for implementing this process and objects having a coating such as obtained by said process.
Zinc surfaces and particularly surfaces obtained by galvanization of ferrous metals are susceptible to corrosion. In particular, during extended use, natural waters form aggressive environments with respect to galvanized objects.
To protect zinc surfaces and particularly surfaces obtained by galvanization, but also, if such be the case, to facilitate certain subsequent treatments, it is known to form on the zinc a protecting surface layer of insoluble phosphates.
This coating, in the techniques known up to present, is provided by contacting the parts with a treating solution containing phosphate ions. The deposit of insoluble salts from this solution results in the chemical shifting of a balance in which the constituents of the solution share of course, but also the metal of the treated surface. In this operating, the parameters which influence the result obtained are multiple. The principal ones are of course bound up with the composition of the solution (nature and concentration), with its pH, but other factors such as the temperature, the duration of the treatment, etc . . . , also play a role.
In general, treating solutions for forming a phosphate protecting deposit contain one or more phosphates soluble in an acid solution. This solution, in contact with the metal surface, produces a slight etching of the metal and the precipitation of insoluble zinc phosphates.
These known techniques are tricky to implement. For satisfactory operation, as we have pointed out, the solutions must have a series of very definite characteristics. Now, the use of these solutions leads rapidly to a modification of these characteristics, so that the user is compelled to check and adjust permanently their composition. In the absence of the right conditions, either the deposit is not formed, or it is accompanied by undesirable phenomena, and particularly by heavy precipitations of sludges which affect the quality of the coating and lead to a wasteful deterioration of the solution.
Another kind of difficulty is bound up with the fact that, for treatments against corrosion, the period of formation of the coating is usually several hours, even several days. The length of the treatment is all the more troublesome since it is most often carried out at temperatures above room temperature and requires permanent heating.
For all these reasons, the known processes are inconvenient to implement and are relatively expensive.
Whatever the measures taken to maintain the treatment solution in the best conditions, the results obtained are not always satisfactory, one of the defects the most often encountered being the lack of uniformity of the coating on a same batch of articles, or even at different parts of the same article. It is known, in the field of anti-corrosion protection, how important it is to provide perfectly uniform coatings without which the corrosion phenomena concentrate at the place where the coating is wanting, thus leading to rapid deterioration of the article considered. One reason for this type of defect is the difficulty in obtaining homogeneous conditions of contact of the solution with the surface to be treated. So, to limit the risk of such defects, efforts are made to ensure agitation of the articles or a flow of the solution, which leads to additional expense for the implementation of these processes.